Optical pickup apparatus for detecting a focus-error signal using a spot size detection method

ABSTRACT

An optical pickup apparatus for detecting a symmetric S-shaped signal to obtain a focus-error signal by an SSD method for stability in operation is provided. In an optical pickup apparatus for irradiating laser light onto an optical recording medium and for directing the light reflected from at least the optical recording medium to a light-receiving device section through a diffraction element to detect a focus-error signal by a spot size detection method using diffracted light caused by the diffraction element, the position of the light-receiving device section is set to the position offset closer to the diffraction element from the focal position of the 0 order light passing through the diffraction element.

TECHNICAL FIELD

The present invention relates to an optical pickup apparatus fordetecting a focus-error signal by a spot size detection (SSD) methodusing diffracted light caused by a diffraction element.

BACKGROUND ART

Optical pickup apparatuses for detecting a focus-error signal by a spotsize detection method (SSD method) using diffracted light caused by adiffraction element are known. Such an optical pickup apparatus isdescribed with reference to FIGS. 6 through 8.

FIG. 6 illustrates an example structure of an optical pickup apparatusfor detecting a focus-error signal by an SSD method.

A laser beam emitted from a laser light source 21 such as a laser diodereaches an objective lens 23 through a beam splitter 22, and isirradiated via the objective lens 23 onto an information recordingsurface of an optical recording medium 10 such as an optical disc.

The light reflected from the optical recording medium 10 returns to thebeam splitter 22 through the objective lens 23, the optical path thereofbeing refracted by the beam splitter 22, and is directed to adiffraction element 24.

The reflected light is divided by the diffraction element 24 into the 0order light passing therethrough, and +1 order light (diffracted light)and −1 order light (diffracted light) diffracted by the diffractionelement 24.

The 0 order light, the +1 order light, and the −1 order light reach alight-receiving device section 25.

In the light-receiving device section 25, for example, light-receivingpatterns shown in FIG. 8 are formed.

A light-receiving device 31 has a light-receiving region E correspondingto the 0 order light.

A light-receiving device 32 has three divided light-receiving regions A,S1, and B, and corresponds to the +1 order light.

A light-receiving device 33 also has three divided light-receivingregions C, S2, and D, and corresponds to the −1 order light.

Each of the light-receiving regions E, A, S1, B, C, S2, and D of thelight-receiving devices 31, 32, and 33 outputs an electrical signalhaving a current level corresponding to the light intensity of theincident light.

The electrical signal output from each of the light-receiving devices31, 32, and 33 is supplied to a matrix amp (not shown) for processing,such as current-to-voltage conversion, amplification, and matrixcalculation, thereby generating a required signal.

That is, a playback signal, focus-error signal, tracking error signal,etc., corresponding to the information recorded in the optical recordingmedium 10 are generated.

The objective lens 23 is held by a two-axis mechanism (not shown) havinga focus coil and a tracking coil so as to be displaceable in thenear-and-apart direction with respect to the optical recording medium 10(focusing direction) and in the direction transverse to the trackorientation of the optical recording medium (tracking direction).

A focus drive signal is generated by a servo circuit (not shown) basedon the focus-error signal to drive the focus coil of the two-axismechanism, so that the objective lens 23 is driven in the focusingdirection so as to be focused with respect to the optical recordingmedium 10.

A tracking drive signal is further generated by the servo circuit basedon the tracking error signal to drive the tracking coil of the two-axismechanism, so that the objective lens 23 is driven in the trackingdirection so as to track with respect to the optical recording medium10.

In the SSD method, the focus-error signal is generated according to thespot size of the diffracted light.

In the focused state shown in FIG. 8( a), the spot size of the +1 orderlight incident on the light-receiving device 32 is equivalent to thespot size of the −1 order light incident on the light-receiving device33.

On the other hand, in the defocused state where the objective lens 23 istoo close to or too far from the optical recording medium 10, as shownin FIGS. 8( b) and 8(c), the spot size of the +1 order light incident onthe light-receiving device 32 is different from the spot size of the −1order light incident on the light-receiving device 33.

Accordingly, by comparing the spot sizes on the light-receiving devices32 and 33, the focus-error signal can be generated.

More specifically, the focus-error signal is generated by, in thesubsequent matrix amp, calculating (A+B+S2)−(C+D+S1) on the outputs ofthe light-receiving regions A, S1, B, C, S2, and D.

In general, when the objective lens 23 moves from the position mostdistant from the optical recording medium 10 to the position closestthereto, as known in the art, in the focus-error signal, a so-calledS-shaped curve shown in FIG. 7 is observed in the vicinity of thefocused position.

A substantially linear region from peak P1 to peak P2 in the curvecorresponds to a so-called in-focus region. In basic operation, when theobjective lens 23 is positioned within the in-focus region, a focusservo controls the position of the objective lens 23 to be brought tothe position of the origin of the S-shaped curve (i.e., the positionwhere focus error=0) based on the focus-error signal.

As shown in FIG. 7, it is assumed herein that the distance of thein-focus region of the S-shaped signal is indicated by d. In otherwords, “d” is defined as the displacement distance of the opticalrecording medium (the distance by which the optical recording mediumchanges with respect to the position of the objective lens) when theS-shaped signal varies from the peak P1 to the peak P2.

Furthermore, one-side in-focus regions d1 and d2 of the S-shaped curvewith respect to the origin of the S-shaped curve are defined as thedisplacement distances of the optical recording medium when the S-shapedsignal goes from the origin of the S-shaped curve to the peaks P1 and P2of the S-shaped curve, respectively. Then, the following equation holdstrue:d=d1+d2  Formula (1)

The origin of the S-shaped curve coincides with the focal position onthe optical recording medium.

This relationship is established, in the standard SSD method, when thediffracted light (the +1 order light and the −1 order light) diffractedby the diffraction element 24 has the same spot diameter r, as shown inFIG. 8( a), resulting in substantial coincidence with the focal positionof the 0 order light (strictly speaking, however, it is shifted towardsthe diffraction element 24 by L·cosθ, where θ denotes the angle ofdiffraction and L denotes the distance between the diffraction element24 and the light-receiving device section 25).

It is assumed herein that the NA (numerical aperture) of the objectivelens 23 is indicated by NA[L]. It is further assumed that the NA of the0 order light in the light focused at the light-receiving device section25 which passes through the diffraction element 24 is indicated byNA[0]. It is still further assumed that the NAs of the +1 order lightand the −1 order light diffracted by the diffraction element 24 areindicated by NA[+1]′ and NA[−1]′, respectively.

It is also assumed that the NAs are so small that the followingapproximation applies: NA=sin θ=tan θ=θ.

Then, the following relationship is obtained:NA[−1]′<NA[+1]′  Formula (2)Thus, the following relationship holds true:NA[−1]′+NA[+1]′=2·NA[L]  Formula (3)

As shown in FIG. 6, the distances from the position of thelight-receiving device section 25 (the focal position of the 0 orderlight) to the focal positions of the diffracted light (the +1 orderlight and the −1 order light) diffracted by the diffraction element 24are indicated by D11 and D12, respectively.

Then, the above-noted one-side in-focus regions d1 and d2 of theS-shaped curve can be approximated as follows:d1={(½)·D11·(NA[+1]′)²}/(NA[L])²  Formula (4)d2={(½)·D12·(NA[−1]′)²}/(NA[L])²  Formula (5)

Since the spot diameters r of the diffracted light on thelight-receiving devices 32 and 33 on the origin of the S-shaped curveare the same, the following equation is obtained:

$\begin{matrix}\begin{matrix}{{r\text{/}2} = {{D11} \cdot {{NA}\left\lbrack {+ 1} \right\rbrack}^{\prime}}} \\{= {{D12} \cdot {{NA}\left\lbrack {- 1} \right\rbrack}^{\prime}}}\end{matrix} & {{Formula}\mspace{14mu}(6)}\end{matrix}$Therefore, the following relationship holds true from Formula (6):D11/D12=NA[−1]′/NA[+1]′  Formula (7)

If NA[0]=NA[+1]′=NA[−1]′ can be approximated, D11 is equal to D12, andthe following equation is obtained from Formulas (4) and (5):d1/d2=1  Formula (8)

Thus, the following relationship is obtained between the diffractedlight (the +1 order light and the −1 order light) and the 0 order light:NA[+1]′=L/(L−D11)·NA[0]  Formula (9)NA[−1]′=L/(L+D12)·NA[0]  Formula (10)

If the distance L between the diffraction element 24 and thelight-receiving device section 25 is sufficiently large, or if thedistances D11 and D12 are sufficiently small, Formula (8) holds true.

If the above-noted approximation does not apply, however, therelationship NA[0]=NA[+1]′=NA[−1]′ does not hold true, and Formula (11)rather than Formula (8) is obtained:

$\begin{matrix}\begin{matrix}{{{d1}/{d2}} = {{\left( {{NA}\left\lbrack {+ 1} \right\rbrack}^{\prime} \right)^{2}/\left( {{NA}\left\lbrack {- 1} \right\rbrack}^{\prime} \right)^{2}} \cdot \left( {{D11}\text{/}{D12}} \right)}} \\{= {{{NA}\left\lbrack {+ 1} \right\rbrack}^{\prime}/{{NA}\left\lbrack {- 1} \right\rbrack}^{\prime}}}\end{matrix} & {{Formula}\mspace{14mu}(11)}\end{matrix}$

In this case, an asymmetric in-focus region of the S-shaped curve isexhibited. That is, the in-focus region shown in FIG. 7 is exhibited.

An asymmetric S-shaped curve means instability in gain of a focus servosignal or an asymmetric focus margin, and is disadvantageous in view ofthe stability in recording to and playback from an optical recordingmedium.

In a device supporting a high-density recording medium, the objectivelens 23 has a high NA. In order to accomplish the same in-focus region dof the S-shaped curve as that described above, as is understood fromFormulas 4 and 5, the focal change distance D11 (D12) with respect tothe diffraction element 24 must increase as the numerical aperture(NA[L]) of the objective lens 23 increases.

This further results in a greater amount of change in the NA of thediffracted light diffracted by the diffraction element 24 than that ofthe 0 order light, as is given by Formulas 9 and 10.

In an optical pickup apparatus which includes the objective lens 23having a high NA, therefore, a more asymmetric in-focus region of theS-shaped curve is exhibited.

Furthermore, desirably, the distance L from the diffraction element 24to the light-receiving device section 25 should be reduced in order toreduce the size of the optical pickup apparatus.

However, as is understood from Formulas 9 and 10, as the distance L fromthe diffraction element 24 to the light-receiving device section 25becomes shorter, the amount of change in the NA of the diffracted lightdiffracted by the diffraction element 24 becomes greater than that ofthe 0 order light. Thus, this case also results in a more asymmetricin-focus region of the S-shaped curve.

As described above, an optical pickup apparatus which obtains afocus-error signal by the SSD method using a diffraction element has aproblem of such an asymmetric in-focus region of the S-shaped curve.

A noticeably asymmetric in-focus region of the S-shaped curve isexhibited, resulting in a large problem, particularly in an opticalpickup apparatus which includes the objective lens 23 having a high NAand in which the distance L from the diffraction element 24 to thelight-receiving device section 25 is small, that is, a compact opticalpickup apparatus used for high-density optical recording media.

DISCLOSURE OF INVENTION

In view of such a problem, an object of the present invention is toprovide an optical pickup apparatus which obtains a focus-error signalby the SSD method, in which a focus-error signal exhibiting a symmetricS-shaped curve can be detected.

To this end, according to the present invention, in an optical pickupapparatus for irradiating laser light onto an optical recording mediumand for directing the light reflected from at least the opticalrecording medium to a light-receiving device section through adiffraction element to detect a focus-error signal by a spot sizedetection method using diffracted light caused by the diffractionelement, the position of the light-receiving device section is set tothe position offset closer to the diffraction element from the focalposition of the 0 order light passing through the diffraction element.

Particularly, the position of the light-receiving device section isoffset closer to the diffraction element from the focal position of the0 order light passing through the diffraction element as long as thefollowing relation is satisfied:(NA[+1]/NA[−1])<(D2/D1)≦(NA[+1]/NA[−1])²

Alternatively, the position of the light-receiving device section isoffset closer to the diffraction element from the focusing position ofthe 0 order light passing through the diffraction element so that thefollowing relation is substantially satisfied:(D2/D1)=(NA[+1]/NA[−1])²

In these cases, NA[+1], NA[−1], D1, and D2 are defined as follows:

NA[+1] indicates the numerical aperture of the +1 order light diffractedby the diffraction element;

NA[−1] indicates the numerical aperture of the −1 order light diffractedby the diffraction element;

D1 indicates the distance from the focal position of the +1 order lightdiffracted by the diffraction element to the light-receiving devicesection; and

D2 indicates the distance from the focal position of the −1 order lightdiffracted by the diffraction element to the light-receiving devicesection.

The light-receiving device section includes a light-receiving devicecorresponding to the +1 order diffracted light, and a light-receivingdevice corresponding to the −1 order diffracted light, each beingdivided into three or five light-receiving regions, and the width of thecenter light-receiving region of the light-receiving devicecorresponding to the +1 order diffracted light is different in size fromthe width of the center light-receiving region of the light-receivingdevice corresponding to the −1 order diffracted light in order tocompensate for a deviation between the focal position on the opticalrecording medium and the position of the origin of the focus-errorsignal.

Particularly, the ratio of the width (s1) of the center light-receivingregion of the light-receiving device corresponding to the +1 orderdiffracted light to the width (s2) of the center light-receiving regionof the light-receiving device corresponding to the −1 order diffractedlight is substantially set as follows:s1:s2=(D1/NA[−1]):(D2/NA[+1])

Alternatively, the ratio of the width (s1) of the center light-receivingregion of the light-receiving device corresponding to the +1 orderdiffracted light to the width (s2) of the center light-receiving regionof the light-receiving device corresponding to the −1 order diffractedlight is substantially set as follows:s1:s2=NA[−1]:NA[+1]

According to the present invention, therefore, the position of thelight-receiving device section is offset closer to the diffractionelement from the focal position of the 0 order light passing through thediffracted light, thereby correcting for an asymmetric shape of thefocus-error signal (S-shaped signal).

Furthermore, a deviation between the focal position with respect to theoptical recording medium and the origin of the S-shaped curve of thefocus-error signal when the light-receiving device section is offset isovercome by making the width of the center light-receiving region ofthree or five divided light-receiving regions of the light-receivingdevice corresponding to the +1 order diffracted light different in sizefrom that of the light-receiving device corresponding to the −1 orderdiffracted light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the structure of an optical pickupapparatus according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a focus-error signal obtained by theoptical pickup apparatus of the embodiment.

FIG. 3 is a diagram illustrating a light-receiving device in alight-receiving device section of the optical pickup apparatus of theembodiment.

FIG. 4 is a diagram illustrating a modification of the light-receivingdevice section of the embodiment.

FIG. 5 is a diagram illustrating a modification of the light-receivingdevice section of the embodiment.

FIG. 6 is a diagram illustrating the structure of an optical pickupapparatus of the related art.

FIG. 7 is a diagram illustrating a focus-error signal obtained by theoptical pickup apparatus of the related art.

FIG. 8 is a diagram illustrating a light-receiving device in alight-receiving device section of the optical pickup apparatus of therelated art.

BEST MODE FOR CARRYING OUT THE INVENTION

An optical pickup apparatus of an embodiment of the present invention isdescribed below.

FIG. 1 illustrates an example structure of an optical pickup apparatusof an embodiment of the present invention for detecting a focus-errorsignal by an SSD method.

A laser beam emitted from a laser light source 1 such as a laser diodereaches an objective lens 3 through a beam splitter 2, and is irradiatedvia the objective lens 3 onto an information recording surface of anoptical recording medium 10 such as an optical disc.

The light reflected from the optical recording medium 10 returns to thebeam splitter 2 through the objective lens 3, the optical path thereofbeing slit by the beam splitter 2, and is directed to a diffractionelement 4.

The reflected light is divided by the diffraction element 4 into the 0order light passing therethrough, +1 order light (diffracted light) and−1 order light (diffracted light) diffracted by the diffraction element4.

The 0 order light, the +1 order light, and the −1 order light reach alight-receiving device section 5.

Particularly in the optical pickup apparatus of this embodiment, thelight-receiving device section 5 is positioned so as to be offset closerto the diffraction element 4 than the related art unit shown in FIG. 6,as is described below.

In the light-receiving device section 5, for example, light-receivingpatterns shown in FIG. 3 are formed.

A light-receiving device 11 has a light-receiving region E correspondingto the 0 order light.

A light-receiving device 12 has three divided light-receiving regions A,S1, and B, and corresponds to the +1 order light.

A light-receiving device 13 also has three divided light-receivingregions C, S2, and D, and corresponds to the −1 order light.

Each of the light-receiving regions E, A, S1, B, C, S2, and D of thelight-receiving devices 11, 12, and 13 outputs an electrical signalhaving a current level corresponding to the light intensity of theincident light.

The electrical signal output from each of the light-receiving devices11, 12, and 13 is supplied to a matrix amp (not shown) for processing,such as current-to-voltage conversion, amplification, and matrixcalculation, thereby generating a required signal.

That is, a playback signal, focus-error signal, tracking error signal,etc., corresponding to the information recorded in the optical recordingmedium 10 are generated.

The objective lens 3 is held by a two-axis mechanism (not shown) havinga focus coil and a tracking coil so as to be displaceable in thenear-and-apart direction with respect to the optical recording medium 10(focusing direction) and in the direction transverse to the trackorientation of the optical recording medium (tracking direction).

A focus drive signal is generated by a servo circuit (not shown) basedon the focus-error signal to drive the focus coil of the two-axismechanism, so that the objective lens 3 is driven in the focusingdirection so as to be focused with respect to the optical recordingmedium 10.

A tracking drive signal is further generated by the servo circuit basedon the tracking error signal to drive the tracking coil of the two-axismechanism, so that the objective lens 3 is driven in the trackingdirection so as to track with respect to the optical recording medium10.

In the SSD method, the focus-error signal is generated according to thespot size of the diffracted light, and, as is similar to the relatedart, the focus-error signal is generated by, in the subsequent matrixamp, calculating (A+B+S2)−(C+D+S1) on the outputs of the light-receivingregions A, S1, B, C, S2, and D.

That is, the principle is also used in which the spot size of the +1order light incident on the light-receiving device 12 and the spot sizeof the −1 order light incident on the light-receiving device 13, whichvary depending upon the focusing conditions (focused/defocused), areutilized.

In this example, however, because of offset of the light-receivingdevice section 5, the spot size of the +1 order light incident on thelight-receiving device 12 is not equivalent to the spot size of the −1order light incident on the light-receiving device 13 in the focusedstate.

Thus, as shown in FIG. 3, the light-receiving device 12 and thelight-receiving device 13 have different sizes. More specifically, thelight-receiving regions S1 and S2 each constituting the center region ofthe three divided light-receiving regions are designed so as to havedifferent widths in the division direction. This meaning is alsodescribed below.

In this example, the focus-error signal exhibiting an S-shaped curveshown in FIG. 2 is observed.

As is understood from comparison between FIG. 2 and FIG. 7, in thisexample, a symmetric S-shaped curve (symmetric S-curve in-focus region)is obtained.

As is similar to the case shown in FIG. 7, it is assumed that thedistance of the in-focus region of the S-shaped signal is indicated byd. In other words, “d” is defined as the displacement distance of theoptical recording medium (the distance by which the optical recordingmedium changes with respect to the position of the objective lens) whenthe S-shaped signal varies from the peak P1 to the peak P2.

In this example shown in FIG. 2 where the symmetry of the S-curvein-focus region is improved, one-side in-focus regions of the S-shapedcurve with respect to the origin of the S-shaped curve are equivalent,and are indicated by d0.

Thus, d0=d/2 is obtained.

A description is made below with terminology defined as follows:

NA[+1] indicates the NA of the +1 order light caused by the diffractionelement 4;

NA[−1] indicates the NA of the −1 order light caused by the diffractionelement 4;

NA[0] indicates the NA of the 0 order light passing through thediffraction element 4;

NA[L] indicates the NA of the objective lens 3;

D1 indicates the distance from the focal position of the +1 order lightdiffracted by the diffraction element 4 to the light-receiving devicesection 5;

D2 indicates the distance from the focal position of the −1 order lightdiffracted by the diffraction element 4 to the light-receiving devicesection 5;

s1 indicates the width of the center light-receiving region S1 of thelight-receiving device 12 corresponding to the +1 order light;

s2 indicates the width of the center light-receiving region S2 of thelight-receiving device 13 corresponding to the −1 order light;

r1 indicates the spot diameter of the +1 order light reaching thelight-receiving device 12; and

r2 indicates the spot diameter of the −1 order light reaching thelight-receiving device 13.

In order to exhibit a symmetric S-curve in-focus region, there is a needfor design in which each one-side in-focus region d0 of the S-shapedcurve is equal to d/2, as shown in FIG. 2.

When the diffracted light (the +1 order light and the −1 order light) isconverted by the diffraction element 4 to NA[+1] and NA[−1],respectively, the focal change distances D1 and D2 of the diffractedlight required for achieving the one-side in-focus region d0(=d/2) ofthe S-shaped curve are given by the following formulas:D1=2d0·(NA[L])²/(NA[+1])²}  Formula (12)D2=2d0·(NA[L])²/(NA[−1])²}  Formula (13)

From the above relationship, the region is determined as follows:

$\begin{matrix}\begin{matrix}{{d0} = {{\left( {1/2} \right) \cdot {D1} \cdot \left( {{NA}\left\lbrack {+ 1} \right\rbrack} \right)^{2}}\left( {{NA}\lbrack L\rbrack} \right)^{2}}} \\{= {{\left( {1/2} \right) \cdot {D2} \cdot \left( {{NA}\left\lbrack {- 1} \right\rbrack} \right)^{2}}\left( {{NA}\lbrack L\rbrack} \right)^{2}}}\end{matrix} & {{Formula}\mspace{14mu}(14)}\end{matrix}$The ratio of the focal change distances D1 to D2 is determined asfollows:D1/D2=(NA[−1]/NA[+1])²  Formula (15)

In the related art, as described in Formula (7), the focal changedistance ratio (D11/D12 ratio) corresponding to the D1/D2 ratio is givenby NA[−1]′/NA[+1]′; whereas, in this example, the light-receiving devicesection 5 is corrected (offset) so as to be closer to the diffractionelement 4 from to the focal position of the 0 order light so that theposition relationship expressed by Formula (15) is established, therebyimproving the symmetry of the S-curve in-focus region.

Thus, while the position relationship of the related art is expressed byD1/D2=NA[−1]/NA[+1], the position of the light-receiving device section5 is offset closer to the diffraction element 4 from the original focalposition of the 0 order light as long as the following relationshipholds true:NA[+1]/NA[−1]<(D2/D1)≦(NA[+1]/NA[−1])²This allows the one-side S-curve in-focus regions of the S-shaped signalto become closer to d0 and d0 shown in FIG. 2 than d1 and d2 shown inFIG. 7. This means that the symmetry of the S-curve in-focus region isimproved.

Since Formula (15), i.e., D1/D2=(NA[−1]/NA[+1])² is derived based ond0θ=d/2, the D1/D2 ratio is substantially equal to the positionrelationship (NA[−1]/NA[+1])² (where the light-receiving device section5 is offset), thereby optimizing the symmetry of the S-curve in-focusregion.

The improvement in symmetry of the S-curve in-focus region achievesstable gain of the focus servo signal or a symmetric focus margin, andtherefore achieves stable recording to and playback from the opticalrecording medium.

As described above, a noticeably asymmetric S-curve in-focus region isexhibited particularly in an optical pickup apparatus supportinghigh-density recording media or a compact optical pickup apparatus. Byoffsetting the light-receiving device section 5 for improvement insymmetry, an optical pickup apparatus suitable for such purposes can berealized.

The spot diameters r1 and r2 of the diffracted light (the +1 order lightand the −1 order light) reaching the light-receiving devices 12 and 13when focused with respect to the optical recording medium 10 aredetermined as follows, respectively:r1=2·D1·NA[+1]  Formula (16)r2=2·D2·NA[−1]  Formula (17)

In the standard SSD method, the diffracted light (the +1 order light andthe −1 order light) diffracted by the diffraction element 4 has the samespot size on the light-receiving devices 12 and 13 when focused withrespect to the optical recording medium 10, and the followingrelationship is obtained:r1/r2=1  Formula (18)Thus, the diffracted light is divided at the same ratio by thelight-receiving devices for signal calculation according to the SSDmethod, so that the origin of the S-shaped curve coincides with thefocal position on the optical recording medium 10.

In this example, on the other hand, when the optical recording medium 10is located at the focal position, the spot diameter ratio of thediffracted light diffracted by the diffraction element 4 on thelight-receiving devices 12 and 13 is given as follows from Formulas (16)and (17), and (12) and (13):

$\begin{matrix}\begin{matrix}{{{r1}/{r2}} = {{{D1} \cdot {{NA}\left\lbrack {+ 1} \right\rbrack}}\text{/}{{D2} \cdot {{NA}\left\lbrack {- 1} \right\rbrack}}}} \\{= {{{NA}\left\lbrack {- 1} \right\rbrack}\text{/}{{NA}\left\lbrack {+ 1} \right\rbrack}}}\end{matrix} & {{Formula}\mspace{14mu}(19)}\end{matrix}$

In this case, there occurs a deviation between the focal position on theoptical recording medium and the origin of the S-shaped curve if thelight is divided on the light-receiving devices 12 and 13 according tothe standard method.

In this example, therefore, as shown in FIG. 3, the width ratio s1/s2 ofthe center light-receiving regions S1 and S2 of the light-receivingdevices 12 and 13 which the diffracted light reaches is defined asfollows:s1/s2=NA[−1]/NA[+1]  Formula (20)ors1/s2=D1·NA[+1]/D2·NA[−1]  Formula (21)

In FIG. 3, the 0, +1, and −1 order light spots are indicated by roundhatched portions on the light-receiving devices 11, 12, and 13,respectively. Since the position of the light-receiving device section 5is offset closer to the diffraction element 4 from the focal position ofthe 0 order light, in the focused state, the spot sizes of thediffracted light irradiated onto the light-receiving devices 12 and 13differ from each other, as shown in FIG. 3. Thus, if the light-receivingdevices 32 and 33 shown in FIG. 8 are used, the focus-error signalobtained by calculating (A+B+S2)−(C+D+S1) is not at the zero level(origin of the S-shaped curve) in the focused state.

Since the ratio of the division-direction widths s1 to s2 of the centerlight-receiving regions S1 and S2 of the three divided regions of thelight-receiving devices 12 and 13 shown in FIG. 3 is designed herein soas to substantially satisfy Formula (20) or (21), the focus-error signalobtained by calculating (A+B+S2)−(C+D+S1) is at the zero level (originof the S-shaped curve) in the focused state. In other words, the lightintensities of the light detected at the light-receiving regions S1 andS2 are equivalent even if the spot sizes differ from each other.

By designing the light-receiving devices 12 and 13 in this fashion, thefocus-error signal can be obtained by calculating (A+B+S2)−(C+D+S1)according to the standard SSD method although the detected spot sizes ofthe diffracted light appear to differ from each other in the focusedstate. Thus, there is no change in design of circuits such as asubsequent matrix amp.

FIG. 4 shows a modification example of the light-receiving devices ofthe light-receiving device section 5. This example indicates that thelight-receiving device 11 corresponding to the 0 order light is dividedinto four detector portions.

As described above, the light-receiving device section 5 is offsetcloser to the diffraction element 4 from the focal position of the 0order light. This also means that the 0 order light incident on thelight-receiving device 11 for the 0 order light has a larger spot size.Thus, even in the case where the light-receiving device 11 is afour-division light-receiving device having light-receiving regions E1,E2, E3, and E4, suitable light detection can be performed in thelight-receiving regions E1, E2, E3, and E4.

Since the light-receiving device 11 is a four-division light-receivingdevice, a so-called push-pull signal or tracking error signal can bedetected by the light-receiving device 11 for the 0 order light.

The push-pull signal is detected as wobble information when the opticalrecording medium 10 is, for example, an optical disc having wobbledgrooves formed therein. In some cases, the tracking error signal may begenerated from the push-pull signal.

Such a push-pull signal is obtained by, for example, calculating(E1+E4)−(E2+E3) on the output of the four-division light-receivingdevice 11.

The four-division light-receiving device 11 can also be used to obtain atracking error signal by a so-called DPD (Differential Phase Detection).In this case, a signal (E1+E3) and a signal (E2+E4) are obtained fromthe output of the light-receiving device 11. The phase error between thesignal (E1+E3) and the signal (E2+E4) is detected to generate a trackingerror signal as a value corresponding to the phase error.

FIG. 5 shows an example where each of the light-receiving devices 12 and13 of the light-receiving device section 5 is divided into five detectorportions.

In this case, the light-receiving device 12 has five light-receivingregions F, A, S1, B, and G. The light-receiving device 13 has fivelight-receiving regions H, D, S2, C, and K.

In the SSD method, in some cases, the five-division light-receivingdevices 12 and 13 may be used to obtain a focus-error signal. In suchcases, the focus-error signal is obtained by calculating{(S1+F+G)+C+D}−{(S2+H+K)+A+B} on the outputs of the light-receivingdevices 12 and 13.

In such cases, also, since the width ratio s1/s2 of the centerlight-receiving regions S1 and S2 is designed so as to substantiallysatisfy above-described Formula (20) or (21) according to the offset ofthe light-receiving device section 5, the focus-error signal obtained bycalculating {(S1+F+G)+C+D}−{(S2+H+K)+A+B} is at the zero level (originof the S-shaped curve) in the focused state.

Originally, such a five-division detector is used for the purpose ofreduction of fake portions F of the focus-error signal shown in FIG. 2.

However, an asymmetric S-shaped curve as exhibited in the related artprevents a reduction of one fake portion. In this example, on the otherhand, the light-receiving device section 5 is offset so as to exhibit asymmetric S-shaped curve, thereby, advantageously, increasing the fakereduction effect by using the five-division detector.

The optical pickup apparatus of an embodiment has been described;however, various modifications may be made to the configuration of theoptical pickup apparatus, particularly to the type or number of opticaldevices, etc.

The optical pickup apparatus of the present invention may beincorporated into a recording device and playback device compatible withvarious optical recording media such as optical discs, magneto-opticaldiscs, and optical cards. Such an optical pickup apparatus is suitableparticularly for incorporation into a device supporting high-densityoptical recording media.

As is understood from the foregoing description, according to theoptical pickup apparatus of the present invention, the light-receivingdevice section is located at the position offset a predetermined amountcloser to the diffraction element from the focal position of the 0 orderlight, thereby improving the S-shaped signal symmetry of a focus-errorsignal detected by the SSD method.

This results in stable gain of the focus-error signal and stablerecording and playback operation of the optical recording medium. TheS-shaped signal having symmetry allows for a symmetric focus margin.Thus, the overall focus margin increases, thereby also achieving stablerecording and playback operation of the optical recording medium.

Another advantage is that the distance between the diffraction elementand the light-receiving device section can consequently be reduced.Therefore, desirably, the size of the optical pickup apparatus isreduced.

The asymmetry of the S-shaped signal is noticeable particularly in anoptical pickup apparatus supporting high-density optical recording mediausing a high-NA objective lens or a compact optical pickup apparatus.However, the asymmetry of the S-shaped signal in such a case can beovercome by the optical pickup apparatus of the present invention, thus,desirably, achieving an optical pickup apparatus for the purpose ofhigh-density recording and playback.

The width of the center light-receiving region of the light-receivingdevice corresponding to the +1 order diffracted light is different insize from the width of the center light-receiving region of thelight-receiving device corresponding to the −1 order diffracted light inorder to compensate for a deviation between the focal position on theoptical recording medium and the position of the origin of thefocus-error signal. There arises still another advantage in that adeviation caused between the focal position with respect to the opticalrecording medium and the origin of the S-shaped curve of the focus-errorsignal even when the light-receiving device section is offset can beovercome, and the focus-error signal can be obtained using a similarcalculation method to the standard method, whereby there is no need fora configuration modification of a subsequent calculation circuit.

1. An optical pickup apparatus for irradiating laser light onto anoptical recording medium and for directing the light reflected from atleast the optical recording medium to a light-receiving device sectionthrough a diffraction element to detect a focus-error signal by a spotsize detection method using diffracted light caused by the diffractionelement, wherein the position of the light-receiving device section isset to the position offset closer to the diffraction element from thefocal position of the 0 order light passing through the diffractionelement, and wherein the position of the light-receiving device sectionis offset closer to the diffraction element from the focal position ofthe 0 order light passing through the diffraction element as long as thefollowing relation is satisfied:(NA[+1]/NA[−1])<(D2/D1)≦(NA[+1]/NA[−1])² where NA[+1] indicates thenumerical aperture of the +1 order light diffracted by the diffractionelement; NA[−1] indicates the numerical aperture of the −1 order lightdiffracted by the diffraction element; D1 indicates the distance fromthe focal position of the +1 order light diffracted by the diffractionelement to the light-receiving device section; and D2 indicates thedistance from the focal position of the −1 order light diffracted by thediffraction element to the light-receiving device section.
 2. An opticalpickup apparatus for irradiating laser light onto an optical recordingmedium and for directing the light reflected from at least the opticalrecording medium to a light-receiving device section through adiffraction element to detect a focus-error signal by a spot sizedetection method using diffracted light caused by the diffractionelement, wherein the position of the light-receiving device section isset to the position offset closer to the diffraction element from thefocal position of the 0 order light passing through the diffractionelement, and wherein the position of the light-receiving device sectionis offset closer to the diffraction element from the focal position ofthe 0 order light passing through the diffraction element so that thefollowing relation is substantially satisfied:(D2/D1)=(NA[+1]/NA[−1])² where NA[+1] indicates the numerical apertureof the +1 order light diffracted by the diffraction element; NA[−1]indicates the numerical aperture of the −1 order light diffracted by thediffraction element; D1 indicates the distance from the focal positionof the +1 order light diffracted by the diffraction element to thelight-receiving device section; and D2 indicates the distance from thefocal position of the −1 order light diffracted by the diffractionelement to the light-receiving device section.
 3. An optical pickupapparatus for irradiating laser light onto an optical recording mediumand for directing the light reflected from at least the opticalrecording medium to a light-receiving device section through adiffraction element to detect a focus-error signal by a spot sizedetection method using diffracted light caused by the diffractionelement, wherein the position of the light-receiving device section isset to the position offset closer to the diffraction element from thefocal position of the 0 order light passing through the diffractionelement, and wherein the light-receiving device section includes alight-receiving device corresponding to the +1 order light diffracted bythe diffraction element, and a light-receiving device corresponding tothe −1 order light diffracted by the diffraction element, each beingdivided into at least three light-receiving regions, and the width ofthe center light-receiving region of the light-receiving devicecorresponding to the +1 order diffracted light is different in size fromthe width of the center light-receiving region of the light-receivingdevice corresponding to the −1 order diffracted light in order tocompensate for a deviation between the focal position on the opticalrecording medium and the position of the origin of the focus-errorsignal, wherein the ratio of the width (s1) of the centerlight-receiving region of the light-receiving device corresponding tothe +1 order diffracted light to the width (s2) of the centerlight-receiving region of the light-receiving device corresponding tothe −1 order diffracted light is substantially set as follows:s1:s2=(D1/NA[−1]):(D2/NA[+1]) where NA[+1] indicates the numericalaperture of the +1 order light diffracted by the diffraction element;NA[−1] indicates the numerical aperture of the −1 order light diffractedby the diffraction element; D1 indicates the distance from the focalposition of the +1 order light diffracted by the diffraction element tothe light-receiving device section; and D2 indicates the distance fromthe focal position of the −1 order light diffracted by the diffractionelement to the light-receiving device section.
 4. An optical pickupapparatus for irradiating laser light onto an optical recording mediumand for directing the light reflected from at least the opticalrecording medium to a light-receiving device section through adiffraction element to detect a focus-error signal by a spot sizedetection method using diffracted light caused by the diffractionelement, wherein the position of the light-receiving device section isset to the position offset closer to the diffraction element from thefocal position of the 0 order light passing through the diffractionelement, and wherein the light-receiving device section includes alight-receiving device corresponding to the +1 order light diffracted bythe diffraction element, and a light-receiving device corresponding tothe −1 order light diffracted by the diffraction element, each beingdivided into at least three light-receiving regions, and the width ofthe center light-receiving region of the light-receiving devicecorresponding to the +1 order diffracted light is different in size fromthe width of the center light-receiving region of the light-receivingdevice corresponding to the −1 order diffracted light in order tocompensate for a deviation between the focal position on the opticalrecording medium and the position of the origin of the focus-errorsignal, wherein the ratio of the width (s1) of the centerlight-receiving region of the light-receiving device corresponding tothe +1 order diffracted light to the width (s2) of the centerlight-receiving region of the light-receiving device corresponding tothe −1 order diffracted light is substantially set as follows:s1:s2=NA[−1]:NA[+1] where NA[+1] indicates the numerical aperture of the+1 order light diffracted by the diffraction element; and NA[−1]indicates the numerical aperture of the −1 order light diffracted by thediffraction element.